This invention relates to the removal of sulfur from hydroprocessed Fischer-Tropsch products.
The majority of fuel today is derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil tends to include nitrogen-containing compounds and sulfur-containing compounds, which are believed to cause environmental problems such as acid rain.
Although natural gas includes some nitrogen- and sulfur-containing compounds, methane can be readily isolated in relatively pure form from natural gas using known techniques. Many processes have been developed which can produce fuel compositions from methane. Most of these process involve the initial conversion of methane to synthesis gas (xe2x80x9csyngasxe2x80x9d).
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes a broad spectrum of products, ranging from methane to wax, which includes a significant amount of hydrocarbons in the distillate fuel range (C5-20). Methane tends to be produced when chain growth probabilities are low. The methane can be recirculated through the syngas generator, but minimizing methane formation is generally preferred. Heavy products with a relatively high selectivity for wax are produced when chain growth probabilities are high. The resulting wax can be hydroconverted to form lower molecular weight products in the distillate fuel and lube base oil range.
The hydroconversion reactions typically include hydrotreatment, hydroisomerization and/or hydrocracking steps, designed to reduce the chain length and/or introduce isomerization. It is often more cost-effective to produce wax products and subject them to hydroprocessing conditions than to form product streams including significant amounts of methane.
The methane used to prepare the syngas used in the Fischer-Tropsch products is typically treated to remove sulfur, since sulfur is a poison for most Fischer-Tropsch catalysts. Accordingly, the products from the Fischer-Tropsch synthesis tend to have relatively low sulfur concentrations. However, the hydroprocessing reactions are often conducted in the presence of sulfur-containing compounds, for example, pre-sulfided catalysts. The hydroprocessing products must be treated to reduce the concentration of the sulfur-containing compounds.
Typically, the natural gas and the hydroprocessing products are treated at separate desulfurizing facilities. This adds to the expense of the overall process. It would be advantageous to provide new methods for removing sulfur from hydroprocessed Fischer-Tropsch products. The present invention provides such methods.
An integrated process for producing desulfurized hydroprocessed products from hydrocarbon synthesis, preferably Fischer-Tropsch synthesis, is disclosed. The process involves treating a well gas and isolating a desulfurized methane-rich fraction, a sulfur rich fraction and a C3+ hydrocarbon fraction. The C3+ fraction comprises an LPG stream (including mainly C3-5 hydrocarbons) and a well gas condensate stream (primarily a C5+ stream). The well gas, derived from a natural gas source, is treated in a treatment zone comprising a first separation zone and a desulfurizing zone.
The desulfurized methane-rich stream is converted to syngas and subjected to a hydrocarbon synthesis step, for example, a Fischer-Tropsch synthesis step. The products from the hydrocarbon synthesis step typically include a C1-C4 fraction, at least one low-boiling liquid fraction (generally in the C5-20 range), and a high-boiling fraction such as wax (C20+). These fractions are isolated in a second separation zone.
The C1-C4 fraction is recycled through the treatment zone, along with the well gas, for isolation of a desulfurized methane-rich fraction for conversion to synthesis gas.
The products from the hydrocarbon synthesis stem tend to be highly linear, and are preferably subjected to additional process steps for upgrading by one or more hydroconversion steps, including hydrotreatment, hydroisomerization, hydrocracking.
The hydroconversion preferably involves contacting the low-sulfur hydrocarbon synthesis products with sulfur-containing compounds such as pre-sulfided catalysts, and/or blending the products with other feed streams, such as petroleum refinery products which include sulfur-containing compounds, such that the hydroconversion products include a relatively higher concentration of sulfur than the hydrocarbon synthesis products. In one embodiment, the sulfur-containing compounds include natural gas liquids, crude oil fractions and/or sulfur-containing compounds derived from crude oil hydroconversion.
The products of the upgrading process are sent to a third separation zone for isolation of at least a gaseous fraction (primarily a C1-C4 fraction), at least one fuel fraction having a predominant fraction boiling in the C5-C20 range, and a heavy fraction which boils predominately in the C20+ range. The C1-C4 fraction can also be sent to the treatment zone and treated in an analogous fashion to the C1-C4 fraction from the hydrocarbon synthesis.
Any sulfur-containing compounds resulting from the additional processing of the hydrocarbon synthesis products in hydroconversion reactions can be routed to the same desulfurization zone used to treat the sulfur-containing compounds in the natural gas, before or after passage through the first separation zone. This eliminates the need for a second desulfurization zone. Since most of the sulfur-containing compounds in the natural gas and hydroconversion products are relatively volatile (i.e., hydrogen sulfide and low molecular weight mercaptans), they will most likely be found in the C1-C4 fractions. The desulfurization zone can be scaled up from its normal size, if desired, to accommodate the additional sulfur removal resulting from the hydroconversion.